Various techniques for reforming heavy oils for use as fuel for the gas turbines have been proposed. There are known, for instance, a method in which the hydrocarbons in a heavy oil are decomposed with supercritical water to convert the heavy oil to a lighter type and a combustible gas is produced therefrom (JP-A-11-246876), and a proposal in which the produced gas is burned to drive a gas turbine while the residue left after oil conversion to a lighter type is burned by a boiler, using the generated heat for producing electric power by a steam turbine (JP-A-11-80750). These techniques may be boiled down to an idea of taking out part of the hydrocarbons in a heavy oil to concentrate the harmful components in the residue.